Method and apparatus for the formation of hot dip coatings



July 22, 1952 M. G. WHITFIELD ET AL 2,604,415

METHOD AND APPARATUS FOR THE FORMATION OF HOT DIP COATINGS Filed Oct. 12, 1948 INVENTORS.

ATTOR NEH Sa Patented July 22, 1952 1 1 VMETHOD ANDVAPPARATUS FOR'THE FOR- v v 'MATION F HOT, DIP COATINGS Marshall G. whitfield Garden City, .andVictor SheshunomJericho, N. .Y., assignors to Whitr *1." I ,Qur invention hasito dowith the'coating of emetab articles with molten dissimilar materials, and relates; particularly to :the coating 'of metal articles i'n strand form, 1. e., Wire, irrespective of its diameter, and strip, irrespective of its .rnaturezof the molten coating :metal likewise'does :notxform' a limitation-zen the-inventiongxand the process :and :apparatus hereinafter described are applicable etc the coating of :metal articles with army ofltheamoltencoating metals employed in thersartifor'thatpurpose, inclusive of zinc, zinc szalloys, aluminum, aluminum alloys, magnesium, magnesium alloys, lead, tin and "terne.

;; .uA principal object of our invention has 'to do with :the provision .of apparatus and a process .for, forming uniform and controlled hot dipped ,coatingson metal articles.

Another princip'alobject of our invention has to do with the formation of heavy controlled coatings-oi thicknesses greater than those hith- 'erto capable'of being controlled asto thickness and uniformity.

In themanufactu're of coated metal products, the metal strand is passed through aba'th of molten coating-metal and carries a layer ofthis metal'with it as it emerges from' the bath. The -coating='-so--carried remains molten on the surface of :the metal article for some time and disi tance-atter 'the article leaves the bath surface. -There 'are various Ways in which some control can-be exercised over the mean weight of coating carried by the metal article out of the bath; but inequalities in the thickness of the :coating are likely to occur through various conditions ;at the 'bath surface and through the'influe'nce ofi gravity wand possibly momentum above the rbath surfac'e while the moltencoating metal is still :soft. :Difierences in uniformity are -.en

- counteredieven with. thin coatings; but theyincoatings ,to attain exceptional corrosion or wear resistance, or-in the case of wire, to-provideimproved electrical conductivity.- It has been pro- .posedin the case of :Wire, coated withzinc to withdraw-the wire; from" --the bath vertically,

field & Sheshunoff, Incorporated, Garden ..City ,N. Yua'corporation of New York i .5, ,-Anp1icatio n.0ctober 12, 1948,S,erial-N0-1; 4l1 2 "17 claims. (01. 117-164) using no Wipers and water-chilling the, coating quickly tokeep it -in-plaoe on the wire... Attempts have beenlmade to withdraw .lboth-vvire and strip at an angle from the -bath. In: some cases for the production of heavy .cLo'atings'fit has :beensuggestedto blow the moltenjmet'al up out-of the bath and ,onto .aninclinedstrip. One of the most serious problems .thehpr'oduc tion of heavy coatings, particularly ,on xwire,.1'.is the need of prompt quenching to maintain the ductility ot the coating and to prevent it. from running-back into the bath. While theqmetaltis' in a liquidor-plastic state,-1-t.is difficult .to-rccol it artificially-without producingroughening-and ins-most instances --.the-,coating -ei-ther -is applied in a non-uniform mayor-becomes.nQn unn rIn immediately upon its emergence from-.theloath. .Light and gheav-yrstriationsin the coating'on strip, and knots orbeadsmf the coatingrnetal, commonly called .ber-ries on wi-re, are familiar to the skilled worker-in the-art; I H Objects of -our "invention are ;the-- solution of the'problemsset'forth above. I I, Another objec zt'ofsonr inventionx-is the provision of apparatus and amethod 'i-n'which the coating applied to the wire or-stripcan'initial which will the set forth .zhereinafter .;or' will -:';be

apparent Z one skilled in the. art upon :readin these specifications, We accomplish :in .that procedure and by that construction: and arrangement of parts of which we :shall :-now :td-escrihe an :exemplary embodiment. Reference :Iisimade to the faccompanying drawings wherein:

Figure l is a diagrammatic section ofaanisaspparatus: for =1coating"metah-strands in accordance Withnuriinvention.

I Fi ure -2 is V a i grammatic :plan :viewmf sth same apparatus. :Figure -3 is a .-di gr ammatic yertical rsectiongiof .asomewhat-modified apparatus.

Briefly in .the 'practice of ,our invention, we Withdraw a-strand-type-artiole. .to be coatedfrom 'a bath of the "molten coating :metal throughsa sizing orifice, but immediately :upon; the;,emergenoe-nf the coatedrarticle,from-,the hathgwe pass it into or through a bath of molten material which is not only inert to the coating metal, but has a. density close to that of the molten metal, so that it can act as a support for the coating while it is still molten. The coating is cooled and solidified while the coated article is in the second bath and prior to its emergence therefrom.

The effect of the elements just described d'epends upon their coaction. effect of a sizing orifice through which the strand is withdrawn cannot be the effect We desire unless the molten coating on the strand is supported immediately beyond the orifice by the inert bath of similar density. Otherwise the molten coating metal will be affected by gravity, and

in the case of horizontal withdrawal fromth'e molten coating bath, will either drip off .the strand or will collect in droplets or heavier and lighter striations prior to its solidification. In the case of vertical withdrawal, the molten metal will run .down. the strand, the general thickness fofjthe coating may be diminished, and non-uniformitywill result. The formation of the familiar' meniscus above the exit rolls in galvanizing operations is a case in point.- On the other hand, 'the use of an inert supporting bath, while it "tends to prevent gravitational effects on the coating while molten and also tends to promote uniformity through surface tension effects, cannot For" example, the j "vertically, it maybe found desirable to employ an inert'bath havinga slightly greater density than thatof the molten coating metal in order toifio'at away from the sizing orifice member any metalor other substance tending to accumulate onthe exit side thereof. a

Fora sizing means we prefer to employ means inthe'nature of a die, made of metallic'or'nom "metallic material of suitable wear resistance and inert both to the 'molten coating metal and to 'the inert bath. The opening in the die will be sufliciently greater in area than the cross-sectional area of the wireor strip being treated to provide for the desired weight of coating. We are not, however, limited to the use of fixed orifice members, especially for strip, since it is possible to employ means such as exit rolls with the strip passing between them, which exit rollsare sealed by suitable doctor means or dams to the wall of the coating pot about the exit orifice thereof.

Referring to Figures 1 and 2, we'have shown at l a coating pot in which a bath 2*of the mol- 'te'n coating metal is maintained Adjacent this pot, and having a wall common therewith, we

have shown a vessel'3 in which the inert bath t is maintained. The strand 5, which in Figures 1 and .2 is illustrated as a wire, passes into and through the bath 2 of molten coating metal'and aboutsuch guiding sheaves B as may be required. It'passs from the bath of molten coating metal into the inert bath 4 through an opening in the common wall, which opening is provided with a die or other sizing means 1. V

The pretreatment which the strand may have received is not a limitation on the invention. We have indicated at 8 the end of a hood element from a pretreatment furnace,1whi'ch end dips beneath the bath of molten coating'metal. One suitable pretreatment is that set forth in the Sendzimir Patent No. 2,110,893, dated March 15, 1938, in which the strand is first passed through an oxidizing furnace in which combustible materials are burned from its surface and in which a thin controlled oxide coating is formed. Next it passes into and through a reducing furnace in which a suitable atmosphere reduces the thin oxide coating, in which the strand is softened by annealing, and which is provided with a cooling hood to reduce the temperature of the strand to slightly above bath temperature before it is introduced into the bath. The index numeral 8 indicates the end of this hood. However, other pretreatments including the passage of the strand through fluxes and the like maybe employed without departing from the spirit of our invention. a I

The inert bath 4 in its vessel 3 may be subjected to heating or cooling or both by various means including the circulation of temperature control fluids through heating or cooling coils 9. If desired, circulation may be maintained in the inert bath 4 and we have illustrated diagrammatically a pump l0 and baflle ll for this purpose. A partial cover I! may be provided for the inert bath. A substantial flow of the inert bath in the directionin which the strand is moving is desirable when high coating speeds are'used.

It will be understood that prior to the emergence of the strand from the inert bath 4, the coating of molten metal thereon will have become solidified. Upon the emergence of the strand, it may be desired to clean from its surfaces any carry-out of the material'o'f the inert bath. We have indicated at It and I4 steam jet devices which may be employed for this purpose. A temperature gradient is maintained in the inert bath from a temperature at the point Where the strand or strands enter it, which will be close to but may be somewhat lower than the temperature of the bath of molten coating metal, to a temperature in remoter portions of the. inert bath which will be lower than the solidification temperature of the molten coating metal. The vessels may be so constructed that heat transfer will occur through the common'wall and the temperature gradient may be maintained by radiation from other parts of the inert bath. However, it is within the scope of our invention to maintain the desired temperature gradient by means of the heating and cooling coils 9 or equivalent apparatus.

'As has already been indicated, the bath 4 should be of a material inert to the molten coating metal and of a density similar to the density of the molten coating metal It should further be of amaterial WhiCh'Wlll be in the liquid phase from a temperature aslhigh as that of the molten coating metal to and below .a lower temperature at which the molten coating metal becomes solidified. Beyond these requirements, however, the material of the inert bath may be any desired. Ordinarily fused salts having the required liquid range and density are chosen. The choice of materials is not confined to fused salts, however.

7 For example, in the coating of steel wire with aluminum, a material available for the inert acoaelt bat-hi is'zmoltensulphur which has a density close to that of molten aluminum. z'Byiwaynflfurther examplesfor aluminum'ooating. aefused salt mixture'comprisingmyolite andalnmina in such pro-1 portionsas to -;give the required zdensityzmay be employed, or a bath of aluminum :fiuorideand barium flu ride in proportion iv'ea mel an..- proachin the density of molten aluminum very closely. For-a zine-coating we may employ anions otherzthingsa bath composed of zinc chlorid and lead oxide or tungsten oxide in, p oporti ns t give-adensity close to or the same as that of molten zinc v q i .a'zl'n 'a particular exemplary. procedure using apparatus like that illustrated in Figures 1 and 2, a steel wire, approximately ,(lGO in.j l-n diameter, was sCQa tQd with aluminum in the pot 1 and can edeto exit intomo'lten sulphur throu h an orifice iapproximatefly mo inzin diameter. T coated wire, cooled while supported bythe sulphlH; jwas found to carry an exceptionally'heavy coating oi approximately 1060 in. inthicknesS which was uniform around the circumference ofgthewire and in. the direction of its length.

@Qur-process affords the opportunity of varying the; thickness of the coating .ofmolten metal to a useful -degree even with -a fixed orifice size. Three-factors appear to govern'the specific thicknessof the coating. These factors are the size of the orifice through which the article to be coated is drawn, the hydrostatic head of the coating metal at.the orifice, and the speed oftravel of thegfitrand-like article. ,In connection with the hydrostatic .head of the molten metal, it will be understood that since'the density of the inert bath-is substantially the same as that of the molten coating..-metal, the. hydrostatic head may be controlled by varying the respective levels of thetwo baths. Thus ,our invention contemplates raising thesurface level of the molten metal coating bath above that of the inert bath when a greater ,thick-nessof the coating and hence a higher hydrostatic head is desired Positive pressure-at the die or orifice may, however, be otherwise-secured, as for example, by coveringthe coating rnetal pot and exerting pressure upon the-surface of the coating metal, as by a gas. The speed of travel of the strand is a factor in the control of coating thickness but is considered a secondary control factor to be tied in with the may-be used .to control to someextent the ,thicknessof the coatingasafiecting the viscosity of the coating metal at the orifice. In operating asi'n the exemplary procedure just outlined, .wehave found it possible to vary the coating thickness from approximately .002. in. to and beyond'the .050 in. previously mentioned.

In Figure 3 we have illustrated a modification of our apparatus .in which we provide a coating .pot l5 with. a sheave I 6 or theliketochange the direction of the strand l! to be coated. -The inert bath in thisinstance,iscontainedin .a stand pipe l8 projecting downwardly into the bath of molten coating metal and, closed at its bottom, excepting for the sizing orifice 19, through which the jstrandpasses. The coatingpot is provided with a cover or closure 20 and with a stand. pipe 2| through which the strand enters the pot. It

ih stajn pipe 8 submerg d. in the ath 2. there 11 besufiicient heat transfer to maintain the-temperature of the} lower partofthe bath .23 atatemperature close-to that of the bath in spite of me irculationtherein. whi a t fi ial 00.1 ing may beep-plied to the upper part of the'bath '3 through t e m um of. coolin oil Z them e of he appar illustrated in F gur .3, it i {may -;-be desirable to employ as an inert bath a mat rialhavine a sli htly grea er density than that-oft. emolt n coatingm tal for t e-PHIPQ-se Qifl a ms'me al, oxide or o her mater al away fromath di orsi i g orifieel9- r '-;An advantag f 7 our pro edu ea d apparatus is that forced or rapid cooling may be employed without destroying the uniformity of the applied coating while the sizing orifice andthe supportingof the coating by the inert bathcoact to providea uniform coating of molten metal which Q is thereafter supported by the inertJoath. until it has solidified. 1 -Modifications may be made in .our' invention withou-t'departing from the spirit of it. Having thus described our invention in certain exemplary embodiments, what we .claimas new and desire to secure by Letters Patent is: v 11.;A process of coating metallic strand-like elemen-ts with molten coatingmetal which com prises passing a strand-likeelement through a.

' bath of molten coating metal andthrough asiaing orifice immediately intoa bath of molten inert substance having a density substantially the same as that of the molten coating metal, whereby to form on the strand a uniform coatingof thev molten coating metal and to support said coating in molten condition immediately after formation, and solidifying said coating While so support a ,2. The process of claim 1 in-which a temperature gradient is maintained in said bath of inert materia1 to cool the coating of molten coating metal to below' its solidification temperature priorto the passage of the coated strand out of the bath of inert material. r

3. The process of claim 1 in which the said bath of inert material is subjected to artificial cooling to chill the molten coatingrmctal rapidly. 4. The process of claim 1 in which the molten coating metal is maintained under conditions .of

, positive pressure at the said orifice greater than the reverse pressure exerted by the bath of inert material. 7

.5. The process of claim 3 in which the molten coating metalis maintained under conditions of positivepressure atthe saidorifice greater than the reverse pressure exerted by the bath of inert material, by maintaining a higher level in said bath of molten coating metal than in said bath of inert material- 6. The process'of'claim 3 in which the molten coating metal is maintained underconditions of positive pressure atthe said orifice greater than the reverse pressure exerted by the bath of inert material, by maintaining a higher, level in said bath of moltenv coating metal than in said bath of inert ,material, and in which the thickness of the coating of molten metal is further controlled by controlling the speed ,of passage of the strand through the said orifice.

.7. A process of coating metallic strand-like and the inertath-zs linthestand ripe-elements l8 and-2 Sinc th i ower end of elements with molten coating metal which comprises passing a strand-like'element through a bath of molten coating metal and through a sizing orificeimmediately into a bath of molten inert' substance of substantially the same density and'contr'olling the thickness of thecoating of molten metal deposited upon the said strand by controlling thegsize of the'said orifice, the speed of travel of the strand and the hydrostatic head of said molten coating metal at said orifice.

8. In coating apparatus a metal coating pot for containing a metal coating bath, a vessel containing a bath of inert material of substantially the same density as the metal coating bath immediately adjacent the said metal coating pot and means "connecting the pot and vessel and comprising a sizing orifice through which a strand to be coated may be passed from the bath of molten coating metal into and through the bath of inert material.

9. In coating apparatus a metal coating pot for containing a metal coating bath, a vessel containing a bath of inert material of substantially thesame density immediately adjacent the metal coatingpot and means connecting the pot and vessel and comprising a sizing orifice through which a strand to be coated may be passed from the bath of molten coating metal into and through the bath of inert material, and means for producing in said bath of inert material a temperature gradient ranging from substantially the temperature of the molten, coating metal adjacent said orifice to a temperature below the freezing temperature of said coating metal at a point remote from said orifice.

10. The structure claimed in claim 8 in which the vessel containing said bath of inert substance is disposed partially within the pot for molten coating metal.

11. The structure claimed in claim 8 in which the vessel containing said bath of inert substance is disposed partially within the pot for molten coating metal, and extends upwardly therefrom as a standpipe, and in which said pot for the molten coating metal is provided with a cover and an upwardly extending standpipe through which said strand enters said bath, whereby the relative pressures of the baths at said orifice may be controlled by controlling the surface levels of thematerials of said baths in said standpipes. L12. The struoture claimed in claim 8 in which the vessel containing said bath of inert substance is disposed partially within the pot for molten coating metal, and extends upwardly therefrom as a standpipe, and in which said pot for the molten coating metal is provided with a cover and anupwardly extending standpipe through which saidstrand enters said bath, whereby the relative pressures of the baths at said orifice may be controlled bycontrolling the surface levels of the, materials of said baths in'said standpipes, the saidorifice being located in the lower part of said first mentioned standpipe vesseland beneath the surface of said molten coating metal.

13. In apparatus for the'purpose' described, a pair of adjacent vessels having a common wall, one ofsaid vessels being a pot for molten coating metal, the other of said vessels containing an inert molten bath of substantially the same density, said common wall having a passageway formed therethrough and sizing orifice means in said passageway whereby a strand-like metallic element to be coated may be led through a molten coating bath in said first vessel and directly 8 through said sizing orifice into said bath of inert material'in said second vessel. I

14. In apparatus for the purpose described, a pair of adjacent vessels having a common wall, one of said vessels being a pot for molten coating metal, 1 the other ofsaid vessels containing an inert molten bath of substantially the same density, said common wall having a passageway formed therethrough and sizingorifice means in said passageway whereby a strand-like metallic element to be coated may be led through a molten coating bath in said first vessel and directly through said sizing orifice into said bath of inert material in said second vessel, and cooling means for said bath of inert material to establish a temperature gradient therein.

15. In apparatus for the purpose described' a pair of adjacentvessels having a common wall, one of said vessels being a pot for molten coating metal, the other of said vessels containing an inert molten bath of substantially the same density, said common wall having a passageway formed therethrough and sizing orifice means in said passageway whereby a strand-like metallic element to be coated may be led through the molten coating bath in said first vessel and directly through said sizing orifice into said bath of inert material in'said second vessel, and cooling means for said bath of inert material toestablish a temperature gradient therein, and means for producing circulation in said bath'of inert material.

16. In apparatus for the purpose described, a pair of adjacent vessels having a common wall, one of said vessels being a pot for molten coating metaL the other of said vessels containing an inert molten bath of substantially the same density, said common wall having a passageway formed therethrough and sizing orifice means in said passageway whereby a strand-like metallic element to be coated may be led through the molten coating bath in said first vessel and directly through said sizing orifice into said bath of inert material in said second vessel, cooling means for said bath of inert material to establish a temperature gradient therein, means for producing circulation in said bath of inert material, and means for cleaning said strand of inert material upon its emergence from the bath thereof.

17. In a process of coating a metal article with a heavy coating of molten coating metal, the steps of passing the article through a bath of molten coating metal whereby to coat it, and while the coating thereon is still molten passing the article into a bath of molten inert substance having a density substantially the same as that of the molten coating metal whereby to support the coating in molten condition and substantially free of gravitational influences so that said coating may become distributed by surface tensional effects, and solidifying said coating while so supported in said inert bath.

MARSHALL G. WHITFIELD. VICTOR SHESHUNOF'F.

' REFERENCES CITED The following references are of record in the file of this patent:

UNITED, s'iATEs PATEuTs 

1. A PROCESS OF COATING METALLIC STRAND-LIKE ELEMENTS WITH MOLTEN COATING METAL WHICH COMPRISES PASSING A STRAND-LIKE ELEMENT THROUGH A BATH OF MOLTEN COATING METAL AND THROUGH A SIZING ORIFICE IMMEDIATELY INTO A BATH OF MOLTEN INERT SUBSTANCE HAVING A DENSITY SUBSTANTIALLY THE SAME AS THAT OF THE MOLTEN COATING METAL, WHEREBY TO FORM ON THE STRAND A UNIFORM COATING OF THE MOLTEN COATING METAL AND TO SUPPORT SAID COATING IN MOLTEN CONDITION IMMEDIATELY AFTER FORMATION, AND SOLIDIFYING SAID COATING WHILE SO SUPPORTED.
 8. IN COATING APPARATUS A METAL COATING POT FOR CONTAINING A METAL COATING BATH, A VESSEL CONTAINING A BATH OF INERT MATERIAL OF SUBSTANTIALLY THE SAME DENSITY AS THE METAL COATING BATH IMMEDIATELY ADJACENT THE SAID METAL COATING POT AND MEANS CONNECTING THE POT AND VESSEL AND COMPRISING A SIZING ORIFICE THROUGH WHICH A STRAND TO BE COATED MAY BE PASSED FROM THE BATH OF MOLTEN COATING METAL INTO AND THROUGH THE BATH OF INERT MATERIAL. 